Stannous ions (Sn2+) are used in oral care compositions because they display biocidal activity and are effective in preventing dental erosion (Rakhmatullina et al, 2013. “Inhibition of enamel erosion by stannous and fluoride containing rinsing solutions”, Schweiz Monatsschr Zahnmed, vol. 123, pp. 192-197). Aqueous solutions comprising stannous ions have been reported to be unstable (Nevitt et al, 1958. “Topical applications of sodium fluoride and stannous fluoride”, Public Health Rep, vol. 73, no 9, pp. 847-850). Stability is an important property to assess when formulating an oral care composition. There is therefore a need in the art for methods of testing oral care compositions for the presence of stannous ions.
Existing methods for analysing oral care compositions include the measurement the total amount of tin present in the composition. However, not all forms of tin are therapeutically active. In particular, stannic (tin (IV)) ions are believed to be inactive. The existing methods do not discriminate between the various forms of tin. There is therefore a need in the art for a method for the selective detection of stannous ions.
WO2008/041055 discloses oral care compositions comprising a stannous ion source, a polyvalent cation source and a mineral surface active agent. A method of investigating the binding of stannous is also outlined. In this method, the stannous is provided in the form of stannous fluoride. The binding of stannous is estimated by potentiometric detection of available ionic fluoride. Hence, this method does not directly detect the presence of stannous ions.